As the current passed through a gas discharge tube is increased beyond the levels at which normal glow discharge takes place, such normal gas discharge being characterized by a negative resistance characteristic leading to decreasing potential between the cathode and anode electrodes of the tube, a region of abnormal glow discharge is entered in which the negative resistance characteristic changes to a positive resistance characteristic leading to increasing potential between the elecytrodes. Typically this increased potential rapidly leads to breakdown into vacuum arc discharge between the electrodes, again characterized by a negative resistance characteristic. Accordingly, gas discharge tubes have been operated in the normal glow discharge or vacuum arc regimes in which stable operation can be achieved by appropriate ballasting of the tube, the former regime being suitable for low current applications and the latter for high current. It is possible to utilize a normal glow discharge tube in a low frequency oscillator circuit by placing capacitance in parallel with the tube and in series with the ballast because such a tube is characterized by a comparatively high striking potential at which discharge is initiated, and a lower but still high extinction potential at which discharge ceases. Operation in such a mode with vacuum arc devices is difficult because, in order to turn off the device effectively, the arc must be extinguished or otherwise interrupted or divested for long enough to disperse the intense ionization formed in its path. On the other hand, the current densities of normal gas discharges are too limited for use in applications requiring relatively large currents.
Devices operating in the vacuum arc regime have other problems, particularly in terms of ensuring adequate electrode life, which have led to gas diodes and triodes (thyratrons) being superseded by semiconductor devices in most applications. A further limitation of such devices is that the great difficulty in turning them off, except by terminating current flow through the device for a finite period, limits their usefulness as control devices to rectification, current turn-on and low frequency alternating current applications.
The only prior art of which we are aware which successfully exploits the abnormal glow discharge regime is the process described in U.S. Pat. No. 3,471,316 (Manuel) issued Oct. 7, 1969, which we understand is commercially utilized in forming organic coatings on metal cans. It relies on the application of externally generated current pulses to force a discharge tube temporarily into the abnormal glow discharge region, the pulses being sufficiently short that no vacuum arc is established. There is no disclosure of any endogenous pulsed abnormal glow discharge, the apparatus is dependent upon an external pulse generator to operate, and its utility is completely different from the present invention because it uses externally generated pulses rather than generating such pulses. U.S. Pat. No. 3,471,316 uses externally generated and limited current pulses to project operation of a discharge tube in a transient manner into the abnormal glow discharge region, thus achieving a higher average current density and accelerating the polymerisation process beyond the rate attainable using a normal glow discharge.